The Concept of Sound Pressure (SPL)
Sound Pressure Level is normally abbreviated SPL. SPL is calculated in decibels or db. Please note that db is a relative term. It can refer to almost anything, however, the most well known is SPL. Most of the time when someone is talking about db, they are probably referring to SPL. The human ear hears pressure, not volume. The difference between sound pressure and volume is that the closer that to the source of the sound, the more pressure you will hear. The farther away you get from the source of the sound, the pressure will be less, but the volume will be the same. A good analogy would be a faucet. At the top of the faucet where the water comes out, there is more pressure than at the bottom, but the volume of water is the same. One thing to remember with SPL is that as you turn the pressure up, you generally also increase the volume. Volume does affect sound, even though it does not directly affect how we perceive how loud something is. SPL can also damage the ear. The human ear starts to degenerate at an SPL of about 85-90 db.
The decibel (abbreviated dB)
The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. The decibel scale is a little odd because the human ear is incredibly sensitive. Your ears can hear everything from your fingertip brushing lightly over your skin to a loud jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That’s a big difference!
On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB. Here are some common sounds and their decibel ratings: Any sound above 85 dB can cause hearing loss, and the loss is related both to the power of the sound as well as the length of exposure. You know that you are listening to an 85-dB sound if you have to raise your voice to be heard by somebody else. Eight hours of 90-dB sound can cause damage to your ears; any exposure to 140-dB sound causes immediate damage (and causes actual pain).
Frequency Weightings — A-Weighted, C-Weighted
The human ear responds more to frequencies between 500 Hz and 8 kHz and is less sensitive to very low-pitch or high-pitch noises. The frequency weightings used in sound level measurements are often related to the response of the human ear, to ensure that these measurements are pretty much what you actually hear.
The most common weighting that is used in noise measurement is A-Weighting. Like the human ear, this effectively cuts off the lower and higher frequencies that the average person cannot hear. Defined in the sound level meter standards (IEC 60651, IEC 60804, IEC 61672, ANSI S1.4), a graph of the frequency response can be seen here:
A-weighted measurements are expressed as dBA or dB(A).
The response of the human ear varies with the sound level. At higher levels, 100 dB and above, the ear’s response is flatter, as shown in the C-Weighted Response to the right. Although the A-Weighted response is used for most applications, C-Weighting is also available on many sound level measurements. C Weighting is usually used for Peak measurements and also in some entertainment noise measurement, where the transmission of bass noise can be a problem.
C-weighted measurements are expressed as dBC or dB(C).
Leq – Equivalent Continuous Sound Level – LAeq
Leq is the preferred method to describe sound levels that vary over time, resulting in a single decibel value which takes into account the total sound energy over the period of time of interest.
A heatmap is a graphical representation of data that uses a system of color-coding to represent different values. Heat maps in Sotto Voce are used in real-time for various forms of visual SPL analytics within the heart of the orchestra.
How loud is it?
*From the Association of British Orchestras*
At present, to our knowledge, There is no central database with representative noise measurements exists. However, from a range of
measurements which have taken place worldwide, we can make the following generalisations:
We know that on a bad day brass players may reach an exposure of 100dB, as may a piccolo. We know that in general the brass have the highest exposures and the fiddles the lowest.
Essentially, the long term averages for symphony orchestras mainly working in Romantic or Contemporary repertoire are: brass about 90dB, woodwind about 88, back strings about 86, front strings about 84.
Chamber orchestras, being less brass-driven, have exposures a couple of decibels lower. Pit orchestras are not significantly higher than symphonic, but by playing a less varied repertoire, have less room for manoeuvre. We know that in general a player’s own instrument dominates their exposure – although another player may drive up the overall playing intensity. We know that a pit is worse (about 3dB) than a stage. We know that repertoire and style are the critical factors. We know that layout is much more important than venue (although the venue clearly affects layout).
Here are several illustrations:
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